Problem Statement Concrete Relaxation Test-Results
Transcript of Problem Statement Concrete Relaxation Test-Results
Effects of Superstructure Time-Dependent Deformations on Bridge Column DesignEbadollah Honarvar Gheitanbaf, Sri Sritharan, and Matt Rouse
Civil, Construction & Environmental Engineering, Iowa State University
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Superstructure Strain Rate
B6 B7 B5
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Pi/A=6.68 Pi/A=6.81
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Estimated Displacement (m)
Uncracked Columns
Testing Testing Procedure
Force control mode
Displacement control mode
Column Test
Two specimen sizes to investigate any plausible size effect
Two various loading scenarios:
i. Instantaneous compressive axial displacement
ii. Step Loading
Beam Test
Precracking loading
Post-cracking loading:
Failed the specimen to examine the possibility of any residual strains in the
rebars
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Load (
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Load/Tensile Strains
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Problem Statement
Analytical Investigation Results
1. Bridge superstructure shortens due to prestressing, and creep and shrinkage
2. Columns are pulled by the superstructure
3. Shear stresses are developed at the column base
Methodology/Objective
Concrete Relaxation Test
Conclusions/ObservationsConcrete relaxation resulted in significant reduction in column base shear force
Generally, the superstructure strain rate increased as the initial axial stress increased
Top of column displacement was estimated fairly accurately using superstructure strain rate when it was compared to the analytical model results
The maximum displacement and base shear force occurred for the most exterior columns, which cracked as well.
The nearest columns to PNM experienced the minimum displacement and base shear force, thus remained uncraked.The nearest columns to PNM experienced the minimum displacement and base shear force, thus remained uncraked.
Concrete Relaxation Test-ResultsFor the seven conduced test, strain remained constant, while force
reduced over time
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Str
ess
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Time (hrs)
Comparison of Compressive Stress Relaxation
Small Column Specimen,
loading age of 48 days
Large Column Specimen,
loading age of 67 days
Small Column Specimen,
loading age of 76 days
Small Column Specimen
Step Loading, loading age of78 days
Small Column Specimen
Step Loading, loading age of84 days
Large RC Beam Precracking,
loading age of 130
Large RC Beam
Postcracking, loading age of150-550
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Time (hr)
Strains for Displacement-Control for 8 in.
Diameter Column
Location1Location
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Loading under Force-Control
ForceDisplacement
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Loading under Displacement-Control
Force Displacement
Analyzed six bridges with
different lengths
Two Short Bridge Frames; Lf < 150 m (492 ft)
Two Medium Bridge Frames
150 m (492 ft) < Lf < 300 m (984 ft)
Two Long Bridge Frames Lf > 300 m (984 ft)
Using the calculated strain
rate, estimated the top of
column displacement with
respect to point of no
movement (PNM)
No residual strains in
longitudinal rebars after
the completion of the
relaxation test
Column relaxation led to
appreciable reduction in
column base shearEstablish a realistic strain rate for the superstructure
shortening
Identify and quantify concrete relaxation
phenomenon
Analyze different bridge frame configurations using the Finite Element Analysis
Propose a simplified design approach to calculate Propose a simplified design approach to calculate column moment/shear demands
Str
ain G
ages
Strain Gages
How to efficiently design bridge columns to accommodate stresses induced by superstructure time-dependent
deformations in posttensioned concrete box-girder bridges